What are the signature features of the reactions of mass publics to terrorist attacks? We argue that the available empirical evidence suggests a general pattern of reactions: The peaks of mass reactions to terrorist attacks are limited in size and duration and their end states marked by a return to baselines values of tolerance. We label this perturbation effects. In this article, we review the available evidence for perturbation effects, build a heuristic model of such effects and provide an explanation of their characteristic pattern, combing theories of emotional arousal and opinion leadership. Finally, we relate the overlooked existence of perturbation effects to widespread fears about the frailty of democratic norms in the face of mass terrorism.